Dopable p-conjugated polymers (alternating double and single bonds along the polymer main chain repeat units), such as those found in the family of polymers known as polyaniline, show potential for a variety of commercial applications such as chemical separations, electromagnetic interference shielding, protection of metals from corrosive environments, antistatic coatings, and current-carrying fibers. Polyaniline is a commercially attractive polymer since, unlike many other dopable p-conjugated polymers, it is both environmentally stable and can be made electrically conducting by acid treatment.
Electrical conductivity (.sigma.) of .pi.-conjugated polymers is physically possible due to electron mobility along (intrachain) and between (interchain) polymer chains in a solid-state article. The magnitude of the conductivity depends upon the number of charge carriers (n) which is determined by the extent of doping with oxidizing or reducing chemical agents (or in the special case of polyaniline, with an acid), the charge on these carriers (q), and on the combined interchain and intrachain mobilities (.mu.). These relationships are related by: EQU .sigma.=n q .mu.
In order to obtain high conductivities, n is usually maximized by a chemical doping process (generation of electrons or holes on the polymer chain), so that conductivity becomes dependent on the mobility of the carriers. At the maximum doping levels, it is the mobility of the charge carriers which must be increased to obtain higher conductivity. Mobility of charge carriers in some cases depends upon the polymer's morphology once it is "frozen" into a nonequilibrium glassy solid state article determined by processing conditions. Interchain mobility depends upon the statistical distribution of conformational features such as bond and torsion angles, interchain distances, packing density, orientation, fractional crystallinity, free volume, etc. On the other hand, intrachain mobility depends upon the degree and extent of .pi.-conjugation and defects along the polymer chains, and the polymer chain conformations. It is therefore desirable to develop improved processing procedures which allow control over the factors governing mobility in order to generate higher conductivities in polyaniline.
Polyaniline in its most useful and environmentally stable oxidation state is given the name emeraldine base (EB). The untreated EB is itself an electrical insulator composed of tetrameric repeating units each containing two secondary amine and two tertiary imine nitrogen atoms as shown in FIG. 1a hereof. When powders of EB are treated with acid solutions, the imine nitrogen atoms extract protons from solution with the acid counterion associating with the polymer chain to maintain overall charge neutrality. When less than 50% of the available imine nitrogens are coordinated to form quaternary iminium salt complexes, i.e., immersion in pH's between 2 and 7, the polymer becomes a semiconductor and is called a bipolaron (See FIG. 1b hereof), since charge carriers delocalized along the .pi.-conjugated polymer backbone are spinless. Immersion in more concentrated acid solutions (pH&lt;2) generates polarons (See FIG. 1c hereof) since, due to self-localized reorganization of electronic states, the mobile charge carriers are now sufficiently delocalized to produce mobile spins. Thus, treatment of EB (which has a conductivity of less than 10.sup.-10 Siemen/cm [S/cm]) with an excess of concentrated acid solution (pH&lt;1) results in an electrically conductive polymer having a conductivity of about 1 S/cm. Under these latter doping conditions, the maximum number of charge carriers (n) have been generated on the polymer since all of the nitrogen atoms, available as protonation sites, are occupied.
The commonly reported polyaniline synthesis describes the heterogeneous radical chain polymerization of aniline at 0.degree. C. in 1 N aqueous HCl, and leads to the acid salt form of polyaniline (See e.g., A. G. MacDiarmid et. al., "Conducting Polymers", Alcacer, L., ed., Riedel Pub., 1986, p.105, FIG. 1c). When this polyaniline powder is immersed in an excess of a strong aqueous base, it is deprotonated to yield EB (See FIG. 1a hereof). Most polyaniline investigations have employed materials having molecular weights with weight average (M.sub.w)&lt;100,000 and number average (M.sub.n)&lt;30,000 which are produced by these synthetic conditions (See, e.g., E. J. Oh et al., "Polyaniline: Dependency Of Selected Properties On Molecular Weight," Synthetic Metals, 55-57, 977 (1993).
In U.S. Pat. No. 5,312,686 for "Processable, High Molecular Weight Polyaniline And Fibers Made Therefrom," which issued to Alan G. MacDiarmid et al. on May 17, 1994, a procedure for preparing high molecular weight polyaniline is reported. The method involves reducing the standard reaction temperature to -30.degree. C., by adding 5 M LiCl to the reaction mixture, thereby producing high-molecular-weight EB. The molecular weight of the resulting polymer may be varied from (M.sub.w)=250,000 to greater than (M.sub.w)=400,000 by controlling the rate at which the initiator is added to the cold reaction mixture, and the reaction temperature. These high molecular-weight polyanilines exhibit poor solubility and have short gelation times. A complex cycling procedure of acid doping, followed by undoping with aqueous base reportedly led to improved solubility and concentrated solutions in N-methyl-2-pyrrolidinone (NMP). Unfortunately these solutions were discovered to rapidly gel when prepared in the 1-3% w/w range in NMP. Thus, there exists a need for developing procedures to process high molecular weight polyaniline.
The utility of polyaniline EB with (M.sub.w)&gt;100,000 and (M.sub.n)&gt;30,000 has been limited. However, in order to process high quality fibers possessing good mechanical properties, it is known in the art that solution concentrations of a particular polymer should be in the 15-30% (w/w) range. Moreover, it is desirable to use the highest molecular weight polymers that will dissolve in solvents in the target concentration range. Tensile strength and modulus, flex life, and impact strength all increase with increasing molecular weight. Typically, molecular weights (M.sub.w)&gt;120,000 and (M.sub.n)&gt;30,000 are preferred. Such solutions are suitable for dry-wet or wet-wet fiber spinning processes that produce high quality fibers, and also for the generation of films, coatings and other useful objects.
The EB form of polyaniline is reported to be soluble in NMP at the 1-5% weight level. Such solutions may be cast into dry dense films after the wet film is thermally treated to remove the solvent. Films prepared in this manner, when immersed in a concentrated acid solution, have a conductivity of between 1 and 5 S/cm. Few other organic solvents for EB, such as N,N,N'N'-tetramethyl urea and N,N'-dimethyl-propylene urea (DMPU) as examples, have been reported in the literature. All of these solvents have carbonyl functional groups, which tend to form strong hydrogen bonds between the carbonyl group of the solvent and the secondary amine groups of EB, thus encouraging limited solubility at dilute concentrations prepared from low molecular weight polymer. However, solubilities of even low molecular weight EB (0.degree. C. synthesis, (M.sub.w)&lt;100,000, (M.sub.n)&lt;30,000) in such solvents is poor (&lt;1-5% w/w). Solutions prepared from NMP above this concentration range exhibit rapid gelation. (See, e.g., E. J. Oh et al., supra). Oh et al. observed that the gelation time is both inversely proportional to the weight percent of EB in NMP and to its molecular weight. S. A. Chen et al. in "Conductivity Relaxation Of 1-Methyl-2-Pyrrolidinone-Plasticized Polyaniline Film", Macromolecules 28, 7645 (1995), have reported evidence for a strong hydrogen bond interaction of the C.dbd.O group from NMP with the secondary amine (NH) functional groups of EB. Presumably, it is the imine nitrogens from the polymer which are strongly attracted to hydrogen atoms of the secondary amines on adjacent chains. This strong attractive force promotes interchain hydrogen bonds which serve as physical cross-links between chains and leads to rapid gelation in EB solutions, or in the solid-state article (FIG. 2a).
Emeraldine base solutions can be processed into free-standing films. If such films are stretched over a hot pin before immersion in a concentrated acid solution, and then subsequently treated with an acid, conductivities of as great as 200 S/cm may be obtained. A. G. MacDiarmid et al., "Towards Optimization of Electrical and Mechanical Properties of Polyaniline: Is Cross-Linking Between Chains the Key?", Synthetic Metals, 55-57, (1993) 753, shows that stretch alignment of EB films [prepared from dilute (1-3% w/w) EB in N-methyl-2-pyrolidinone (NMP) solutions], over a hot pin at 120.degree. C. to a 2-5.times. draw ratio, increases the films fractional crystallinity (from .about.5 to 50%) and additionally increases the anisotropic conductivity of the maximally acid doped film from 1 to 200 S/cm, in the direction parallel to the stretch. Hence, this example demonstrates the importance of manipulating the parameters which control carrier mobility (.mu.) in the solid-state articles to enhance physical properties such as conductivity.
Some researchers have reported preparation of EB solutions having &gt;10% w/w from DMPU (See e.g., K. T. Tzou, R. V. Gregory, "Improved Solution Stability And Spinnability Of Concentrated Polyaniline Solution Using N,N-DimethylPropylene Urea As The Spin Bath Solvent" Synthetic Metals 69, 109-112, 1995). Here also, the investigators employed a synthetic procedure which yields low molecular weight EB ((M.sub.w)&lt;100,000, (M.sub.n)&lt;30,000). The solutions were stable long enough for the authors to spin a fiber which exhibited high conductivity; however, the details of processing and the solubility limits are lacking, and the resulting mechanical properties of the fiber would be much improved if higher molecular weights were accessible in their solvent systems.
A second category of reported solvents for polyaniline includes acids, such as m-cresol, formic acid, methanesulfonic acid, sulfuric acid, as examples. Solubility derives from the basic nature of the EB polymer which forms ionic coordination complexes between the acid and the imine nitrogens of the polymer. Solubility increases as the strength of the acid increases (&gt;10% w/w for sulfuric acid, 1-5% w/w in m-cresol and formic acid). It is doubtful that EB is truly dissolved in such acid solutions; rather, it is more likely that the solutions consist of a fine dispersion of polyaniline particles. Processing EB in such solutions is not desirable since 1. The solvents are hazardous; 2. Strong acids can either over-oxidize emeraldine or chemically substitute on the polymer rings; and 3. The resulting polymers tend to degrade if stored in solution for more than a few days. Additionally, even though partially soluble in acid media, EB fibers spun from acid solution have been found to be mechanically weak.
A major obstacle to the fabrication of commercially useful articles, such as high quality fibers, hollow fibers, or articles having other useful geometries, from solutions of polyaniline, therefore, is the poor solubility of the polymer in solvents suitable for processing using conventional polymer engineering methods. Such solutions exhibit a strong tendency to form gels on a relatively short time scale due to interchain hydrogen bond formation, even for dilute solutions. The instability is such that the solutions cannot be extruded through spinnerette orifices because they gel too rapidly or form particulate material which clogs the spinnerette tip, causing unsafe pressure increases in the spin line which represent a significant health risk to operators.
U.S. Pat. No. 5,135,682, for "Stable Solutions Of Polyaniline And Shaped Articles Therefrom, which issued to Jeffrey D. Cohen and Raymond F. Tietz on Aug. 4, 1992, discloses a procedure for preparing stable dry-wet spinning solutions of EB in the 10-30% w/w range. Stable, spinnable solutions were prepared using 1,4-diaminocyclohexane, 1,5-diazabicyclo (4.3.0) non-5-ene, or by dissolving EB in NMP with the addition of specified quantities of cosolvents consisting of either pyrrolidine (Py) [11% EB; 33% Py; and 56% NMP w/w/w] or ammonia. The amount of pyrrolidine added as cosolvent, compared to the amount of the EB added to NMP solution, can be expressed as the ratio of moles Py/moles EB tetrameric repeat unit, which in their preferred embodiment is 15.5. (The molecular weight of the EB repeat unit is 362 g/mol, and that of Py is 71.13 g/mol). Poor quality fibers were observed for the NMP/Py solutions (See, e.g., ibid., Example 5). The work was further described in "Polyaniline Spinning Solutions and Fibers," by C. -H. Hsu, J. D. Cohen and R. F. Tietz, in Synthetic Metals 59, 37 (1993), where the authors suggested that the physical degradation of the polyaniline fibers, especially after exposure to an acid, was likely due to the addition of Py or ammonia cosolvents, as a result of chemical interactions between the cosolvent and the polymer. Molecular weights reported from the described synthetic procedure were approximately (M.sub.n)=20,000 and (M.sub.w)=120,000. Synthetic conditions were carried out at -8.degree. C. without LiCl added to the reaction mixture.
In U.S. Pat. No. 5,147,913, for "Cross-Linked Polymers Derived From Polyaniline And Gels Comprising The Same," which issued to Alan G. MacDiarmid and Xun Tang on Sep. 15, 1992, the preparation of cross-linked polymers of polyaniline by providing a substantially linear polymer which comprises polyaniline and/or a polyaniline derivative, admixing the linear polymer with a liquid in which the cross-linked polymer is substantially insoluble, and cross-linking the polymer through agitation, is described. Preferred liquids for preparing such gels include NMP. A preferred embodiment for forming such gels is utilization of EB in NMP at concentrations &gt;5% w/w.
In "Stabilization of Polyaniline Solutions," by Debra A. Wrobleski and Brian C. Benicewicz, Polymer Preprints 35, 267 (1994), the authors report the addition of hindered amine antioxidants and UV absorbers to up to 5% w/w solutions of EB in NMP to increase the gelation time for such solutions. Although molecular weights for the EB are not reported, the described synthesis must have produced EB with weight average molecular weights below (M.sub.w)&lt;100,000 and number averages (M.sub.n)&lt;30,000.
Accordingly, it is an object of the present invention to provide a method for dissolving high concentrations (between 15% and 30% w/w) of high molecular weight polyanilines (weight averages (M.sub.w)&gt;120,000 and number averages (M.sub.n)&gt;30,000) without significant gel formation over a time period sufficient to process the solution obtained thereby into articles.
Another object of the invention is to provide a method for preparing solutions having high concentrations (between 15% and 30% w/w) of high molecular weight polyanilines ((M.sub.w)&gt;120, 000 and (M.sub.n)&gt;30, 000) from which articles can be prepared having improved electrical conductivities and mechanical properties.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.